Many sensors output a signal in response to a given environmental condition. For instance a thermocouple has a voltage output due to a temperature difference from one end to the other end of the sensor. Thermocouples are analog temperature sensors that utilize the thermoelectric properties of two dissimilar materials, typically metals, to generate an EMF in proportion to a temperature gradient across a material inhomogeneity. Common thermocouples used in temperature measurement comprise two metal wires of different thermoelectric properties called thermoelements connected at one end to form a “hot junction” also known as a “measuring junction”. The other ends of the wires are connected to instrumentation such as a voltmeter to measure the EMF produced by the thermocouple. The wires are connected to the instrumentation at a known reference temperature to form a “reference junction” or a “cold junction”.
Thermocouples and other such sensors follow a nominal response curve to allow the user to quantify the measured parameter based on the output of the sensor. Most of the sensors will have a shift in output over time from an initial response curve. This irreversible change may be due to chemical or metallurgical changes in materials used to construct the transducer. This shift in the output of the sensor is commonly referred to as drift. Since it is desirable to make as accurate a measurement as is feasible it would be advantageous to the user of the sensor if the drift could be eliminated. In many instances it is not easy to eliminate drift.
Sensor drift can be caused by many different mechanisms but two prime sources of drift are chemical or metallurgical changes. For instance a common drift mechanism in type K thermocouples, where the chromium preferentially oxidizes, is a phenomenon called “green rot”. By chemically binding some of the chrome in an oxide the percent composition of chrome in the alloy is essentially reduced causing a shift in the thermoelectric output of the sensor. Green rot has been found to be difficult to prevent from occurring.
Not all sources of drift are due to chemical or metallurgical changes. For instance a sensor may become mechanically damaged causing a shorted condition leading to a false reading or a sensor may become water soaked leading to electrical shunting and apparent drift. Such occurrences may not be correctable and are difficult to predict or reproduce in a lab environment. While these other sources of drift are no less important in attaining accurate measurements it is still desirable to correct for as many sources of drift as possible. In addition, the level of drift may vary greatly between manufactured lots of sensors due to variations in the chemical and/or metallurgical properties of the materials used in the sensors. There are also sources of drift that are temporary and reversible such as the temperature effects on a pressure sensor. Thus, drift may be thought of as having two basic origins: drift produced by reversible effects such as those due to ambient temperature, and drift caused by irreversible changes due to fundamental alterations in the chemical or metallurgical properties of the transducer.
Accordingly, there is a need for compensating for sources of drift that may be predicted and reproduced in the lab for thermocouple systems and other such sensor systems while mitigating variations in the sensor output due to drift between material lots used in the manufacturing of the sensors.